But it will damage your iPhone 5 and reduce its overall battery capacity.

Completely false.

The power supply has a maximum amount of power that it can deliver, but that power isn't constant or mandatory. 5W for the compact chargers that come with iPhones, 10W for the older iPad chargers and 12W for the newer ones. But, an iOS device will only take the maximum amount of current it can handle.

Bottom line: the iPad charger won't harm your iPhone 5 in any way. It will charge slightly faster, but won't max out the iPad charger's power output, and won't hurt your battery.